The present invention relates to a thixotropic forming process suitable for wheels fashioned from rheocast metal alloy in the semisolid state, and in particular for the manufacture of aluminium alloy road wheels fitted ultimately with pneumatic tyres. The shaping processes employed conventionally in the manufacture of road wheels for pneumatic tyres are essentially those of forging, and casting in permanent molds, both of which well known in the wider field of mechanical engineering.
Forging is the familiar process by which a metal alloy can be shaped in the solid state. Employed in the particular context of the art field pertinent to the manufacture of wheels for pneumatic tyres, this is a facility which allows the realization of products with superior mechanical properties, but which at the same time gives rise to a number of drawbacks, namely, the need to use alloys suitable for plastic working, the impossibility of producing articles with geometrically complex shapes, and the fact that the end product will be arrived at only after implementing a series of consecutive steps, especially a product characterized by significant variations in thickness such as are evident in the typical geometry of a road wheel. The high cost of the forging process represents a further drawback. The process of casting in a permanent mold, where an alloy is worked in the liquid state, allows the realization of a product at low cost in relatively few steps, and in this instance even with complex geometries. By contrast, the mechanical quality of the cast product is inferior to that of the forged product, and, moreover, with casting no less than with forging, there is the need to utilize alloys having particular intrinsic properties specifically suited to the technological process in question. More especially, the lower mechanical quality of the cast wheel is attributable to the structural characteristics of casting alloys, as well as to the porosity and discontinuity which are generated within the fabric of the wheel and derive from the particular type of casting process.
In addition, both of the processes mentioned above are characterized in that the forged or cast piece requires generous allowances of material, dictating the need for extensive additional machining steps before the piece can be considered an end product.
Recent times have seen the development of a new technology, namely the thixotropic forming of metal alloys in the semisolid or semiliquid stated; in this instance, the end product is obtained from an ingot or billet exhibiting a particular structure that appears physically homogeneous on macroscopic inspection, but when viewed microscopically appears as a plurality of solid globular granules immersed in a liquid phase. The ingot can take on different characteristics according to the percentages, by weight, of the solid and liquid fractions: in the case of a semisolid, the material can behave in the manner of a solid, for example when conveyed from a heating station to a work station or thixotropic injection forming station, but in the manner of a liquid when injected under pressure.
There are currently no known applications of this new technology in the art field that embraces the manufacture of wheels for pneumatic tyres.
Given that there are clear advantages and drawbacks alike with both the forging process and the casting process conventionally adopted in the manufacture of road wheels, as intimated above, the object of the present invention is to overcome the drawbacks of each such method while combining the advantages.